1. Field
Aspects of the present invention relate to a multilevel power converter, particularly to pulse width modulation control of a multilevel converter.
2. Description of the Related Art
Traditionally, multilevel power converters are used in the applications of medium voltage AC drives, flexible AC transmission systems (FACTS), and High Voltage DC (HVDC) transmission systems, because single power semiconductor devices cannot handle high voltage. Multilevel converters typically include a plurality of power cells for each phase, each power cell including an inverter circuit having semiconductor switches that are capable of altering the voltage states or levels of the individual cells. Depending on the type of inverter circuitry used (for e.g., half-bridge or full bridge), each power cell may have one or more switching legs. By controlling the switching events of the individual switching legs of each power cell, it is possible to control the voltage across each cell and resultantly obtain an AC output waveform having multiple discrete voltage levels. A multilevel converter is often described by the number of discrete levels in output voltage waveform.
In certain applications, it may be desirable to control the switching events in a multilevel converter using Pulse Width Modulation (PWM). A PWM based control provides several benefits, especially a reduction in the harmonic spectrum at every level. Multilevel converters typically use phase-shifted triangular carriers at the heart of the PWM method. A conventional method used for multilevel converters, particularly those having a cascaded H-bridge topology, is phase-shifted pulse width modulation (PS-PWM) carrier method. In the PS-PWM method, a reference signal for a particular cell, which is typically a sine-waveform, is compared against a triangular carrier in order to obtain the switching instances for a first switching leg of the cell. Each cell has its own triangular carrier. In the PS-PWM method these carriers are phase-shifted. The same reference sine-waveform is compared against the inverted triangular carrier in order to obtain the switching instances for the second switching leg of the same cell.
But the conventional methods, such as those mentioned above, do not provide an optimum spectrum for the line-line output voltage. The quality of the output voltage deteriorates especially at high output voltage frequency, or when the converter has a low number of levels. Typically if the output voltage frequency is high and the converter has a reduced number of levels, an obvious option is to increase the switching frequency. But increasing the switching frequency also increases the overall losses.